Variable light attenuators in the path of an emitted laser beam or other solid state light source enable efficient and stable operation of the light source while attenuating the light intensity to a suitable level for its application. A number of different approaches have been used for variable light attenuator design, including the use of spatially varying neutral density (ND) filters and other absorptive devices. One conventional method for variable light attenuation, as shown in FIG. 1A, employs the well known Malus' law. In this conventional arrangement, a laser attenuator 10 receives light from a laser light source 12, directed through a half-wave plate 14 and to a polarizing beamsplitter 16 or other polarizing element. Variable rotation of the half-wave plate about the optical axis enables a variable amount of the laser light to be transmitted through the polarizing element, with the remaining light directed to a light dump element 20 from the polarizing beamsplitter or absorbed by the polarizer.
Other attempts to develop variable laser light attenuators using multilayer thin-film technologies have proved disappointing. Among approaches that have been proposed is the use of a set of multilayer dielectric coated substrates having various reflectivity values. Each substrate is designed for a specific wavelength and provides a reflectance value for incident light of that wavelength. Installed at a slight angle to the beam, the attenuator reflects back some proportion of the light, as determined by the arrangement of dielectric layers.
One reason for the limited success of thin-film approaches relates to polarization. Practitioners in the optical arts have generalized the definitions of two mutually orthogonal polarization states as follows: light that has its polarization axis parallel to the plane of incidence is defined as having polarization state P, or P-polarized light; light that has its polarization axis perpendicular to the plane of incidence is defined as having polarization state S, or S-polarized light. For polarized light, the relative phase and amplitude of its P- and S-polarized components is fixed or constant. For un-polarized light, the relative phase and/or amplitude of P- and S-polarized components is random.
Conventional wisdom in thin-film design, reinforced by numerous practical examples, holds that multilayer thin-film surfaces are not polarization-neutral when light is not incident at a normal to the surface, but rather exhibit noticeable differences in their handling of light having different polarization axes. This principle is exploited in various designs, such as in the design of various types of polarizing beamsplitters. By way of illustration, FIG. 1B shows an exemplary transmission spectrum of a polarizing beamsplitter formed using thin-film coatings for two orthogonal polarization states. For one polarization state, shown as P-polarization, transmission is very high between wavelengths λ1 and λ2; for the other, S-polarization, most of the light in this range is reflected. A number of polarizing beamsplitter designs employ this same principle for separating light of different polarization states, over a given wavelength band.
Even where some attempt has been made to control the behavior of the different polarization states in a thin-film filter design, it has proved difficult to provide similar handling of light in S- and P-polarization states. By way of example, FIG. 1C shows the spectral characteristic of a high-performance edge filter having a relatively steep edge. Curves for P-polarized light P and S-polarized light S are shown, along with a curve for polarization-averaged light A, that is, the average of P- and S-polarized light. Transmission is shown from 0 dB (0 OD) to −100 dB (10 OD), over a range of angles of incidence (AOI) from 0 to 20 degrees. A difference curve D, shown as a dashed line and with its scale at the right, indicates the difference between attenuation for S- and P-polarized light at each point. As can be seen from FIG. 1C, this filter design provides nearly equivalent attenuation of both S- and P-polarized light within only a very limited range of angles, in the range from about 2 to 3 degrees. The attenuation then changes dramatically from that point, so that by about 4 degrees, the difference in attenuation as shown in curve D is about −5 dB (0.5 OD). Over an AOI range from greater than 2 to about 9 degrees, polarization-averaged attenuation ranges from 0 OD (0 dB) to 4 OD (−40 dB), as shown by curve A. However, over this same 7 degree angular range, the difference between attenuation to S— polarized light and attenuation to P-polarized light ranges from near 0 dB to about −16 dB (0 OD to 1.6 OD).
This problem in handling S- and P-polarized light differently has long been recognized by those skilled in thin-film filter design. As just one acknowledgement of the inherent problems caused by polarization, researchers Gu and Zheng, in an article entitled “Design of non-polarizing thin film edge filters” in Journal of Zhejiang University SCIENCE A (2006) 7(6) pp. 1037-1040, note the difficulty in dealing with polarization differences in thin-film response and state that “the separation between S- and P-polarization components is an invariable effect in such interference thin film edge filters at non-normal light incidence.” In addition, a number of patents and related publications also attest to this apparently inherent, invariable behavior, and some considerable effort has been expended in thin-film component design to try to correct or compensate for differences in handling light components of different polarization states. For example, U.S. Pat. No. 4,373,782 entitled “Non-Polarizing Thin-Film Edge Filter” to Thelen describes different behavior for light having P (parallel) and S (perpendicular) polarization axes and proposes aligning a single edge of an interference bandpass filter, at a fixed incident angle, to achieve similar response for P- and S-polarized light at a single wavelength or over a very narrow band of wavelengths. Other attempts try to address the problem of differences in how light of different polarization axes are handled by adjusting the position of polarization peaks for the different P- and S-polarization states, as described in U.S. Pat. No. 5,926,317 entitled “Multilayer Thin Film Dielectric Bandpass Filter” to Cushing, and in U.S. Patent Application No. 2003/0128432 entitled “Polarization Independent Thin Film Optical Interference Filters” by Cormack et al., for example.
A similar approach to that proposed in the Cushing '317 disclosure, matching polarization response for a single wavelength and fixed angle, is adapted for color separators and combiners by researchers Chen and Gu in an article entitled “Design of non-polarizing color splitting filters used for projection display system” in Displays 26 (April 2005) pp. 65-70. However, a combination of high dynamic range, high levels of attenuation, and insensitivity to polarization is not achieved, nor would it be useful with a spectral combiner or separator using such a coating. Notably, what these researchers teach provides neither linear attenuation response over a range of incident angles, nor similar attenuation for both S- and P-polarizations over a range of angles.
It is generally accepted that there is no way to surmount this problem; polarization differences are considered to be simply inherent to devices formed using layers of isotropic thin-film materials. In an article entitled “Nonpolarizing and polarizing filter design” in Applied Optics, 20 Apr. 2005, authors Qi et al., note that “changes in phase thicknesses and in optical admittances of the layers are fundamental and cannot be avoided at oblique incidence.” These authors also note that, as a result, different reflective and refractive properties are exhibited for the transverse-electric and transverse-magnetic fields of a plane wave in dielectric thin films. Their proposed solution for avoiding polarization-dependence is to use birefringent (i.e., nonisotropic) thin-film designs, which are considerably more difficult to fabricate than their isotropic counterparts.
Inherent differences in how thin-film coatings handle light having orthogonal polarization axes and experience with thin-film coatings in various filter applications offer little promise for non-polarized laser attenuation using these coatings. One example of a proposed solution is given in U.S. Pat. No. 4,778,263 entitled “Variable Laser Attenuator” to Foltyn that describes the use of a matched pair of thin-film interference filters arranged at different angles to provide variable laser attenuation. However, the Foltyn '263 disclosure clearly indicates a high degree of polarization dependence as well as wavelength dependence for an attenuation device formed in this manner, intended for use with unpolarized excimer lasers. Moreover, as is shown in FIG. 8 of the Foltyn '263 disclosure, the average light output is non-linear and may not even be monotonic over a range of incident angles.
Thus, there is a long-felt need for a variable laser attenuator that provides attenuation over a range of values and is polarization-neutral over that range.